Polymeric materials have a wide variety of uses, especially in the areas of absorbent articles and disposable items. As used herein, the term "absorbent articles" refers to devices which absorb and contain body exudates and, more specifically, refers to devices that are placed against or in proximity to the body of the wearer to absorb and contain the various exudates discharged from the body. The term "absorbent articles" is intended to include diapers, training pants, absorbent underpants, incontinence products, bandages and the like. The term "disposable" is used herein to describe articles not intended to be laundered or otherwise restored or reused, such as protective articles including industrial work wear and medical garments such as surgical drapes and gowns, as well as other articles of clothing either as the entire article or simply as a component thereof.
Thermally and solution-processed elastomers are commonly used in disposable personal care absorbent articles for waist bands, side panels and other gasketing applications. In the case of waist bands and side panels, the elastomer functions to hold the article in place during and under actual use conditions. Actual use conditions typically involve heat, humidity, loading or any combination thereof. In the case of gaskets, the elastomer functions to hold the disposable personal care absorbent article closer to the wearer's skin and thereby prevent leakage. The gaskets also manage some stress and prevent sagging of the absorbent article over a certain period of time. Some of these elastomers, however, lose their elastic properties after a period of exposure to actual use conditions.
Lycra strand is an excellent elastomer and performs well at body temperature. The superior elastic properties result from the well-defined, phase-segregated molecular structure of Lycra strand. In general, Lycra strand is a two-phase polymer system having a hard phase and a soft phase. The phase segregation occurs between the hard and soft rubber segments which constitute the backbone of the polymer. Lycra strand behaves almost like an ideal elastomer because of the substantial difference in the glass transition, or melting point, of the rubber block and the glass transition of the urea, hard segment. Lycra strand, however, is expensive because (1) it is produced by wet spinning and (2) there is a lack of competitive products on the market.
There are other block copolymers based on urethane or ester that attempt to take advantage of the concept of phase segregation. These copolymers include polyurethane elastomeric materials such as, for example, those available under the trademark ESTANE.RTM. from B. F. Goodrich & Company of Akron, Ohio and those available under the trademark MORTHANE.RTM. from Morton International, Inc. of Seabrook, New Hampshire and polyester elastomeric materials such as, for example, those available under the trade designation HYTREL.RTM. from E. I. Dupont de Nemours & Company of Wilmington, Del. and those known as ARNITEL.RTM., formerly available from Akzo Plastics of Arnhem, Holland and now available from DSM of Sittard, Holland. Such urethane and ester copolymers, while capable of delivering excellent elastic properties, can be costly. They are, therefore, not always cost effective for use in the business of disposable personal care absorbent articles.
Thermoplastic block copolymer elastomers based on blocks of polystyrene and rubber blocks of isoprene, butadiene, siloxane, amorphous ethylene, propylene or a combination of amorphous ethylene and propylene, are attractive for use in disposable personal care absorbent articles because of the lower cost, the ease of processing and the non-toxicity. However, at body temperature, these copolymers lose their elastic properties and dimensional stability after a period of time and begin to sag and leak. The loss of elastic properties and dimensional stability appears to be associated with the polymer itself. The segments of polystyrene begin to flow and slip at the normal human body temperature, especially under tension or load. The motion of the styrene blocks adversely affects the elastic character of the polymer, resulting in the loss of dimensional stability and elastic properties.
A need, therefore, exists for a cost-effective polymer that retains its elastic properties while also having increased mechanical properties and that is useful personal care absorbent articles such as diapers, training pants, incontinence devices and the like.
Similarly, in protective articles such as industrial work wear and medical garments, the motion of the styrene blocks adversely affects the elastic character of the polymer, resulting in the loss of dimensional stability and shape. A need, therefore, exists for a polymer that imparts improved in-use durability to such protective articles.